The usefulness of the phenomenon of magnetostriction in linear distance or position measuring devices is recognized by the prior art; for example, see Redding, U.S. Pat. No. 4,305,283; McCrea et al, U S. Pat. No. 4,158,964; Krisst, U.S. Pat. No. 4,071,818; Edwards, U.S. Pat. No. 4,028,619; and Tellerman, U.S. Pat. No. 3,898,555. A magnet near or around the magnetostrictive wire marks the location to be measured. Such devices can operate with either mechanical or electrical excitation. When an acoustical/mechanical strain propagating along the wire reaches the area of influence of the magnet an electrical signal is generated. Conversely, when an acoustical/mechanical strain propagating along the wire reaches the area of influence of the magnet an electrical signal is generated. Such linear position detectors are utilized as liquid level detectors. The position of the magnet, and hence the liquid level, is determined as a function of the time required for an acoustical/mechanical disturbance to propagate from one end of the wire through the area of influence of the magnet in the case of mechanical excitation or from the position of the magnet to a sensing apparatus located at one end of the wire in the case of electrical excitation.
An improvement on such devices is disclosed by Dumais U.S. Pat. No. 5,017,867 which includes a reflective termination at the foot of the magnetostrictive wire and measures the difference of the propagation times of a pulse from the magnet position to the foot of the wire and reflected back to the head of the device and of a pulse traveling directly from the magnet to the head. This technique provides twice as much resolution of each measurement since the reflected pulse travels twice as far as the direct pulse for each increment of magnet displacement.
In the field of liquid level detection, it is often useful to simultaneously measure liquid level and measure liquid temperature at one or more locations. Many liquids change volume with temperature. Thus a measurement based upon level alone would not distinguish between cases where the mass of liquid had changed and where the mass of liquid is the same but the volume has changed due to a temperature change. Tellerman, U.S. Pat. No. 4,726,226 has proposed a combined apparatus for simultaneously detecting liquid level using a magnetostrictive position detecting apparatus and detecting temperature at a plurality of positions within the liquid via temperature dependent resistors. Tellerman, U.S. Pat. No. 4,726,226, teaches an encoding technique for transmitting both position and temperature information to a remote site using a single pair transmission line. The resistances of the temperature dependent resistors are measured and these values are used to vary the period of a pulse generator. Position measurements are made at the varying pulse periods of the pulse generator. A composite signal is transmitted on the transmission line in the form of a series of pulses. The time between certain non-consecutive pulses is a measure of the liquid level. The time between groups of pulses corresponds to one of the temperature measurements. The sequence of temperature measurements is known to the apparatus receiving the signal via the transmission line, enabling the pulse period to be translated into temperature. Tellerman, U.S. Pat. No. 4.726,226, further teaches the use of two precision temperature independent resistors one having a resistance less than the range of the temperature dependent resistors and one having a resistance greater than this range. These fixed resistances provide fixed pulse periods enabling absolute calibration and correction for any component drift.
A similar system was proposed in U.S. Pat. No. 5,050,430 which differs from Tellerman, U.S. Pat. No. 4,726,226 in providing plural liquid level/temperature measurements in sequence. The combined apparatus produces a composite signal for transmission on a two wire transmission line including information regarding the linear displacement measured and the temperature measured by each temperature dependent resistor. The resistance of the temperature dependent resistors is measured in a predetermined sequence employing a sequential switching circuit. The resistance of a first reference resistor having a temperature independent resistance which is less than the lowest expected resistance of the temperature dependent resistors is first measured. Next, the resistance of a second reference resistor having a temperature independent resistance which is greater than the highest expected resistance of the temperature dependent resistors is measured. Then, the resistances of the temperature dependent resistors are measured in a predetermined sequence.
The major drawback of these techniques is that in both cases the time required to transmit the complete set of level and temperature data is rather long. Since usually a single secondary system controls up to 8 probes, and the system switches from one probe to another only after it has collected a certain number of level and temperature readings, it is highly desirable to minimize the time for reading each probe. The possibility of minimizing this time using each of the aforementioned approaches is limited due to different reasons.
When the first technique is used, the shortest temperature-encoded interval (corresponding to the lower end of the temperature range if RTD's or positive temperature coefficient thermistors are used as temperature sensors, or to the upper end if negative temperature coefficient thermistors are used) must be longer than the longest possible interrogation period. In order to provide for good resolution of the temperature measurements the longest temperature-encoded interval must be 2-3 times longer than the shortest interval.
An additional reason that the interval cannot be minimized is related to the following circumstance. Most of the liquid level probes are required to be intrinsically safe which means that the flow of energy into the probe is severely limited. On the other hand a noticeable momentary power is required to interrogate the magnetostrictive wire in order to achieve better accuracy, especially if a solid magnetostrictive wire is used. The only possible way to resolve this contradiction is to accumulate energy in a capacitor - a process that takes time. In practice, the minimum periodicity of level interrogation is about 9 milliseconds.
Another drawback is that the remote controller which receives data from the probe and a number of other probes must have a special logic circuit for each of the time intervals to be measured. Where there are two level measurements--one for water in a container and one for a liquid product above the water--and a temperature measurement, a counter in the controller must be turned on for the interval between the interrogation pulse and the first signal to measure product level, between the interrogation pulse and second signal to measure water level, and between two interrogation pulses to measure temperature related values.
Still another problem is that to synchronize into the serial signal sequence the controller must accumulate a significant number of measurements and analyze the data. In one commercial system a minimum of 128 measurements must be made. If a pulse is missed or added due to noise, etc., all this data will be lost and the process must be repeated.